The present invention relates to a metal-oxide semiconductor (MOS) transistor and a manufacturing method thereof, and more particularly, to a MOS transistor adopting a titanium-carbon-nitride (TiCN) gate electrode, and to a manufacturing method thereof.
A MOS transistor comprises a semiconductor substrate, source/drain regions formed by ion-implanting an impurity into the substrate, a channel region between the source/drain regions, and a gate formed over the channel region by intervening a gate insulating film. Here, the impurity type for the source/drain regions is opposite that for the substrate. Generally, the gate electrode of the MOS transistor is formed of N type impurity-doped polysilicon, which exhibits stable processing characteristics and high sheet resistance.
Meanwhile, in a PMOS transistor, which operates as a buried channel device, it is difficult to reduce threshold voltage without increasing short channel effects. As a way to overcome this problem, research efforts have concentrated on manufacturing a gate electrode from a metal material having a lower resistivity than polysilicon. Titanium nitride (TiN) has been especially suggested as a suitable gate electrode material.
Since a TiN gate electrode possesses a resistivity as low as 50 .mu..OMEGA.-cm or below, the RC delay time can be reduced. Another advantage is that the TiN gate electrode enables both an N-channel MOS transistor and a P-channel MOS transistor to operate as surface channel devices, since titanium nitride has the midgap energy level (E.sub.f) of silicon. Hence, degradation of the transistor characteristics caused by short-channel effects can be prevented and process simplicity can be achieved.
In practice, however, the actually attained Fermi energy level of a fabricated TiN gate electrode deviates somewhat from the midgap energy level of silicon, according to the structure and chemical composition of the TiN film. This phenomenon will be described in connection with FIGS. 1 and 2.
FIG. 1 compares the capacitance-versus-voltage characteristics of the conventional MOS transistors employing a polysilicon gate electrode (plot a) and a TiN gate electrode (plot b). Here, it can be seen that the difference between the work functions of the TiN gate electrode and the polysilicon gate electrode is 0.6-0.7 eV. This indicates that the Fermi energy level of a TiN gate electrode shifts from the midgap energy level of silicon to the valance band.
FIGS. 2 and 3 are graphs showing the variation of threshold voltages depending on the gate length of conventional PMOS and NMOS transistors having a TiN gate electrodes, respectively. As the Fermi energy level of the TiN gate electrode shifts from the midgap energy level of silicon to the valance band thereof, its built-in potential increases, leading to an increase in the threshold voltage of the NMOS transistor and a decrease in that of the PMOS transistor.
As described above, the Fermi energy level of the TiN gate electrode deviates from the midgap energy level of silicon. This deviation creates a short channel, thereby deteriorating the characteristics of a transistor.